The base system of the present invention employs an Orthogonal Frequency Division Multiplexing (OFDM) method. The transmission data assigned to each channel is temporally divided and then transmitted, and since the transmission intervals of OFDM symbols are increased through a serial/parallel converter, the effect of multiple paths can be reduced.
FIG. 1 is a transmission block diagram based on Orthogonal Frequency Division Multiplexing (OFDM). Data streams to be transmitted to the respective second communication stations are converted through channel encoders 101, 104 and 107. The above channel-encoded symbols, respectively, pass through interleavers 102, 105 and 108 and are then modulated by data modulation blocks 103, 106 and 109. The transmission time of data that have undergone data modulation is determined through a Time-Division Multiplexing (TDM) block 110, and a selected data symbol stream is carried on an OFDM-based subcarrier along with pilot information 111. A process of carrying the data symbol stream on the subcarrier is performed through a serial/parallel converter 112, an Inverse Fast Fourier Transform (IFFT) block 113, and a parallel/serial converter 114. A Cyclic prefix 115 for eliminating the interference between adjacent subcarriers, and the interference between adjacent OFDM symbols is inserted into the OFDM symbols that have passed through the parallel/serial converter 114, and the resulting OFDM symbols are transmitted through a transmission block 117 via a digital-analog converter 116.
A multi-carrier spread spectrum system is a system in which a conventional Code Division Multiple Access (CDMA) method and the OFDM method are combined together. The second communication stations are respectively assigned unique orthogonal codewords, and spread and transmit transmission data using the assigned orthogonal codewords. The subcarriers used for transmission are also assigned by the first communication station.
FIG. 2 is a transmission block diagram of a multi-carrier spread spectrum system. The overall procedure of converting respective data bit streams, to be transmitted to second communication stations, up to data modulation blocks 203, 206 and 209, is the same as those of the orthogonal frequency division multiplexing method shown in FIG. 1. Data that have undergone data modulation is multiplied by an orthogonal codeword from an orthogonal code generator 210, and each of the respective data symbol streams to be transmitted to the second communication stations is assigned a transmission time and a subcarrier to be used at the time of transmission through a multiplexer and data symbol mapper 211. The subsequent processes are the same as those described in FIG. 1.
In the multi-carrier spread system, the data symbol streams in FIG. 2 are classified into three types according to the operation of the multiplexer and data symbol mapper 211.
FIGS. 3, 4 and 5 show examples of the three operations of the multiplexer and data symbol mapper.
In FIG. 3, data symbol mapping is performed such that data symbol streams 304, 305, 306 and 307 respectively multiplied by orthogonal codewords are arranged along a frequency axis 302 and are transmitted. In a subcarrier group 309 assigned according to the size of a orthogonal codeword (for example: Nc=8), the respective data symbol streams to the second communication stations are multiplexed on an orthogonal code axis 301 and are transmitted. In this case, a data symbol stream, which is spread using a single orthogonal codeword, is mapped to be transmitted for an OFDM symbol interval 308, that is, for a symbol time interval of Ts. Since the system has Nsub subcarriers in total, there exist [Nsub/Nc] subcarrier groups.
In FIG. 4, data symbol mapping is performed such that data symbol streams 404, 405, 406 and 407, respectively multiplied by orthogonal codewords, are arranged on a time axis 403 and then transmitted. The data symbol streams use only a single subcarrier, and are multiplexed along an orthogonal code axis 401 so as to be transmitted for Nc (for example: Nc=8) OFDM symbol intervals 408, that is, for a symbol time interval of Ts. Since FIG. 4 shows an example of a case for Nc=8, the transmission of data symbol streams is performed during 8 symbol intervals.
In FIG. 5, data symbol mapping is performed such that data symbol streams 504, 505, 506 and 507, respectively multiplied by orthogonal codewords, are arranged on both of a frequency axis 502 and a time axis 503 and then transmitted. The data symbol streams are multiplexed on an orthogonal code axis 501 so as to be transmitted for Nt (for example: Nt=4, 510) OFDM symbol time intervals Ts 508, in a single subcarrier group 511 that is composed of Nf subcarriers 509. The size of the orthogonal codeword assigned to the system is Nc, so that Nc is equal to the multiplication of a spreading factor Nf 510 in the frequency axis and a spreading factor Nt 509 in the time axis. In the system, the total number of subcarriers is Nsub, and there exist, [Nsub/Nf] subcarrier groups.